1. Field of the invention
The present invention relates to an inverter apparatus, and more particularly to an inverter apparatus with an adaptable voltage-to-frequency control.
2. Description of the Prior Art
An induction motor is commonly driven in a scalar control method, a vector control method, or a direct torque control method. The principle of the scalar control method is to change synchronous speed of the induction motor by changing input frequency of the induction motor. The scalar control method is also called a voltage-to-frequency control (V/f control) method, or a variable voltage variable frequency control (VVVF control) method. In general, the V/f control method is an open-loop control method, namely, a rotational speed of the induction motor is easily changed by using an inverter without feeding back the rotational speed. However, torque of the induction motor will reduce because output frequency of the inverter increases while input voltage of the inverter is not simultaneously changed. Hence, in order to keep magnetic flux of the induction motor constant to generate maximum efficiency, the ratio of voltage magnitude to operation frequency has to be a constant value, namely, the voltage-to-frequency control (V/f control) method is so called.
Reference is made to FIG. 1 and FIG. 2, wherein the FIG. 1 is a structure block diagram of a prior art inverter apparatus, and the FIG. 2 is a block diagram of converting an analog input voltage into an output frequency of the prior art inverter apparatus. The inverter apparatus 1A comprises a conversion circuit 10A and a micro-controller unit 20A. The conversion circuit 10A includes a first gain unit 10A, a DC-offset unit 102A, and a second gain unit 103A. The first gain unit 101A provides a first voltage gain P1a (P1a=+0.5) to transform the analog input voltage Vin (Vin equals −10 to +10 volts) into a first gain voltage Va (Va equals −5 to +5 volts). The DC-offset unit 102A provides a +5-volt DC-offset voltage Vdc′ (Vdc′=+5 volts) and is connected to the first gain unit 101A to generate a modified voltage Vx (Vx equals 0 to +10 volts). The second gain unit 103A provides a second voltage gain P2a (P2a=+0.5) to transform the modified input voltage Vx (Vx equals 0 to +10 volts) into an analog output voltage Vo (Vo equals 0 to +5 volts). The micro-controller unit 20A includes an analog-to-digital converter unit 201A and a frequency operation unit 202A. The analog-to-digital converter unit 201A converts the analog output voltage Vo into a corresponding digital output value, and the frequency operation unit 202A generates a corresponding output frequency according to the digital output value.
A relation between a voltage variation ΔV of the analog input voltage Vin and the analog output voltage Vo of the inverter apparatus 1A is shown as following:ΔV=(10−(−10))/(5−0)×0.1=0.4 (volts)
Namely, the micro-controller unit 20A can receive the analog output voltage Vo in 0.1 volts when the analog input voltage Vin is at least changed in 0.4 volts. Hence, the inverter apparatus 1A can not provide a high-resolution voltage variation to accurately control a drive apparatus.